The European Union's top justice official warned yesterday that any EU country found to have operated secret CIA prisons could lose its EU voting rights.

In a move that increases pressure on the US to explain the activities of the CIA, the EU justice and home affairs commissioner, Franco Frattini, said there would be "serious consequences" if reports of CIA jails in Europe turned out to be true.

"I would be obliged to propose to the council [of EU ministers] serious consequences, including the suspension of voting rights in the council," Mr Frattini said at a counter-terrorism meeting in Berlin.

Mr Frattini's remarks follow claims that the CIA set up a covert network of "black sites" in eastern Europe in the wake of September 11. Earlier this month Human Rights Watch claimed that the agency secretly interrogated al-Qaida suspects at Soviet-era jails in Poland and Romania. Both countries deny it. Poland's president, Aleksander Kwasniewski, reiterated yesterday in a broadcast by news station TVN24 that his country has never hosted any CIA prisoners.

The foreign secretary, Jack Straw, has urged the Bush administration to clarify the issue amid further reports that secret CIA planes carrying terrorist suspects regularly stopped off in several European countries. Germany's new foreign minister, Franz-Walter Steinmeier, will raise the subject today during his first visit to Washington. His symbolic fence-mending trip to the US has been overshadowed by new allegations that the CIA used Germany as a hub for its "rendition" flights. During the flights Islamist prisoners were allegedly transferred to third countries where they could be interrogated beyond the reach of international human rights legislation.

Last week the Berliner Zeitung reported that at least six CIA planes had touched down at Frankfurt's Rhein-Main air base. The claims are embarrassing for Angela Merkel, Germany's new chancellor, who wants to repair relations with the US. Several European governments have launched their own investigations into clandestine CIA prisoner-transfer landings.

The European commission has also asked the US to confirm the existence of secret CIA military jails, which almost certainly breach the European convention on human rights and the international convention against torture.

A European council investigator, Dick Marty, has said he wants to examine satellite photos of a military training camp at Szymany in north-eastern Poland and Kogalniceanu military airport in southern Romania.

Some 31 CIA flights are under suspicion, he has said. If the allegations are proved, Poland, a US ally which joined the EU last year, and Romania could be in breach of article 6 of the Treaty of Nice, which calls on all member states to uphold basic human rights. The Bush administration has so far not confirmed the existence of secret CIA prisons.

Yesterday Mr Frattini said the voting rights suspension would be justified under the EU's treaty, which stipulates that the bloc is founded on the principles of liberty, democracy, respect for human rights, fundamental freedoms and the rule of law. Jonathan Faul, chief of the EU's justice and home affairs directorate, last week formally raised the issue of alleged CIA secret detention centres with the White House and state department representatives, who told him Washington needed more time to respond to the media allegations, Mr Frattini said.